Fast-swimming hydromedusae exploit velar kinematics to form an optimal vortex wake

Fast-swimming hydromedusan jellyfish possess a characteristic funnel-shaped velum at the exit of their oral cavity that interacts with the pulsed jets of water ejected during swimming motions. It has been previously assumed that the velum primarily serves to augment swimming thrust by constricting the ejected flow in order to produce higher jet velocities. This paper presents high-speed video and dye-flow visualizations of free-swimming Nemopsis bachei hydromedusae, which instead indicate that the time-dependent velar kinematics observed during the swimming cycle primarily serve to optimize vortices formed by the ejected water rather than to affect the speed of the ejected flow. Optimal vortex formation is favorable in fast-swimming jellyfish because, unlike the jet funnelling mechanism, it allows for the minimization of energy costs while maximizing thrust forces. However, the vortex `formation number' corresponding to optimality in N. bachei is substantially greater than the value of 4 found in previous engineering studies of pulsed jets from rigid tubes. The increased optimal vortex formation number is attributable to the transient velar kinematics exhibited by the animals. A recently developed model for instantaneous forces generated during swimming motions is implemented to demonstrate that transient velar kinematics are required in order to achieve the measured swimming trajectories. The presence of velar structures in fast-swimming jellyfish and the occurrence of similar jet-regulating mechanisms in other jet-propelled swimmers (e.g. the funnel of squid) appear to be a primary factor contributing to success of fast-swimming jetters, despite their primitive body plans

Morphological diversity of medusan lineages constrained by animal–fluid interactions

Cnidarian medusae, commonly known as jellyfish, represent the earliest known animal taxa to achieve locomotion using muscle power. Propulsion by medusae requires the force of bell contraction to generate forward thrust. However, thrust production is limited in medusae by the primitive structure of their epitheliomuscular cells. This paper demonstrates that constraints in available locomotor muscular force result in a trade-off between high-thrust swimming via jet propulsion and high-efficiency swimming via a combined jet-paddling propulsion. This trade-off is reflected in the morphological diversity of medusae, which exhibit a range of fineness ratios (i.e. the ratio between bell height and diameter) and small body size in the high-thrust regime, and low fineness ratios and large body size in the high-efficiency regime. A quantitative model of the animal–fluid interactions that dictate this trade-off is developed and validated by comparison with morphological data collected from 660 extant medusan species ranging in size from 300 µm to over 2 m. These results demonstrate a biomechanical basis linking fluid dynamics and the evolution of medusan bell morphology. We believe these to be the organising principles for muscle-driven motility in Cnidaria

Algebraic characterization of gauge anomalies on a nontrivial bundle

We discuss the algebraic way of solving the descent equations corresponding to the BRST consistency condition for the gauge anomalies and the Chern--Simons terms on a nontrivial bundle. The method of decomposing the exterior derivative as a BRST commutator is extended to the present case.Comment: 15 pages, LaTeX, no figure

Flow patterns generated by oblate medusan jellyfish: field measurements and laboratory analyses

Flow patterns generated by medusan swimmers such as jellyfish are known to differ according the morphology of the various animal species. Oblate medusae have been previously observed to generate vortex ring structures during the propulsive cycle. Owing to the inherent physical coupling between locomotor and feeding structures in these animals, the dynamics of vortex ring formation must be robustly tuned to facilitate effective functioning of both systems. To understand how this is achieved, we employed dye visualization techniques on scyphomedusae (Aurelia aurita) observed swimming in their natural marine habitat. The flow created during each propulsive cycle consists of a toroidal starting vortex formed during the power swimming stroke, followed by a stopping vortex of opposite rotational sense generated during the recovery stroke. These two vortices merge in a laterally oriented vortex superstructure that induces flow both toward the subumbrellar feeding surfaces and downstream. The lateral vortex motif discovered here appears to be critical to the dual function of the medusa bell as a flow source for feeding and propulsion. Furthermore, vortices in the animal wake have a greater volume and closer spacing than predicted by prevailing models of medusan swimming. These effects are shown to be advantageous for feeding and swimming performance, and are an important consequence of vortex interactions that have been previously neglected

The Social Security Cost of Smoking

Our paper is an examination of the Social Security cost of smoking from an individual point of view. It is well known that smokers have a shorter life expectancy than nonsmokers. This means that by smoking they are giving up potential Social Security benefits. We estimate this cost and consider the effects on the system as a whole. We use mortality ratios, which relate the annual death probabilities of smokers and nonsmokers, and the percentage of smokers in each age group to break down the life tables for men and women born in 1920 into the approximate life tables for smokers and nonsmokers. We then calculate expected Social Security taxes and benefits for each group, using median earnings as a base. We find that smoking costs men about $20,000 and women about$10,000 in expected net benefits. The implication of this for the system as a whole is that the prevalence of smoking has a direct effect on the financial viability of the system; every decrease in the number of smokers in society increases the system's liability. Changes in smoking behavior should be recognized as affecting the system.

PRICE AND THE STRUCTURE OF FREIGHT CAR OWNERSHIP

Demand and Price Analysis,

Ion sputter-deposition and in-air crystallisation of Cr2AlC films

Ternary alloys of composition close to Cr2AlC have been deposited by ion beam sputtering onto unheated and heated to 380 °C Si substrates. As-deposited films are amorphous. Annealing of the film in vacuum at 700 °C leads to crystallisation with 39.2 nm crystallites. Crystallisation also can be achieved by annealing in air but there is also partial oxidation of the film surface to the depth of approximately 120 nm, which represents an oxide layer less than 5% of the total film thickness. There is an increase of lattice size along the c-axis during crystallisation in air, which indicates a small incorporation of oxygen. Film structure and crystallisation have also been analysed by Raman spectroscopy. Changes in Raman spectra in Cr2AlC have been correlated with the film crystallisation and it was observed that MAX-phase related peaks become clearly defined for the crystallised film